Modulated carrier wave transmitter



April 23, 1940 W.T.D1TCHAM 2,198,431

IODULATED CARRIER WAVE TRANSMITTER Filed lay 1, 1937 2 Sheets-Sheet '1] p Sol/RC5 0F WAVEEIIEREY M0004 A TING POTEN TIA L 5 INVENTOR. v

1447.01 (HAM BY V65 ATTORNEY G a I u W. T. DITCHAM Filed llay 1, 1937 MODULATED CARRIER WAVE TRANSMITTER INVENTOR W7. BITCH/1M BY 744 MV A/ ATTORNEY AAAAAAAA II'IVIV" A n'l zs, 1940.

Patented Apr. 23, 1940 PATENT OFFICE MODULATED CARRIER WAVE TBANSDHTTEB William Theodore Ditcham, London, England, assignor to Radio Corporation of America, a corporation of Delaware Application May 1, 1937, Serial No. 140,155 In Great Britain July 21, 1936 5 Claim.

This invention relates to radio and other modulated carrier wave transmitters and has for its object to provide improved efflcient and simplifled modulating circuit arrangements whereby a radio or other high frequency carrier may be modulated in accordance with speech, music, television, or other signals.

According to this invention a modulating circuit arrangement for modulating a radio or other high frequency carrier wave comprises a carrier frequency thermionic tube stage; a carrier frequency load circuit for said stage; a modulating tube stage in series with said load circuit; means for applying carrier frequency energy from said first mentioned tube stage across a series circuit including in series said modulating tube stage and at least a part of said load circuit; means for applying energy of carrier Wave frequency and of substantially constant amplitude to the input electrodes of said modulating tube stage; and means controlled in dependence upon modulating potentials for varying the phase relation between the carrier energy applied to said input electrodes and that applied across said series circuit and/or for varying the time duration 1. e., the pulse width" of the carrier energy applied to said input electrodes. Preferably the first mentioned tube stage is an amplifier stage driven in well known way at carrier frequency and having a tuned anode circuit and preferebly also the load circuit is a tuned circuit galvanically or otherwise coupled to said tuned anode circuit in such manner that there is a voltage step up between said tuned anode circuit and said tuned load circult.

The invention is illustrated in the accompanying drawings which show diagrammatically one embodiment.

In the drawings Figure 1 is a simplified circuit illustrating dia-; grammatically the essential elements of my modulation system; While Figure 2 is a more detailed showing of the system of Figure 1. In Figure 2 the amplifier stage or first stage is neutralized.

Referring to the drawings carrier frequency energy is applied-for example through a coupling condenser I from a master oscillator source 2-across a resistance 3 connected between control grid '3 and cathode 5 of a carrier amplifier triode 6 whose cathode is connected to ground and to the negative terminal I of a source (not shown) of anode potential and whose anode 8 is connected through a choke 9 to the positive terminal it of said source. The anode 8 is con- (Cl. 1'I9171.5)

nected through coupling .condenser II to a tapping point l2 on the inductance i3 01' a parallel tuned oscillatory circuit l3, l4, one end of which is connected to the anode I5 of a modulating tube l6 and the other end of which is connected 5 to the cathode 5. The modulating tube l6 (as shown a triode) has its cathode ll connected to one end of a load circuit in the form of a parallel tuned circuit l8, IS the inductance l8 being coupled to an aerial or other utilization circuit 20. This tuned circuit l8, I9 is grounded at its other end. Oscillations from the source 2 are fed via a variable phase adjusting network 2| of any suitable kind known per se between control grid 22 and cathode ll of the modulating tube 16, the connection to the control grid 22 including a coupling condenser 23 adjacent said grid. Modulating potentials are applied in any manner known per se, for example via a transformer 24 to vary the phase shift introduced by the 20 phase adjusting network 2i. The control grid 22 of the modulating tube It is connected to the cathode I! through a choke 25 in series with a negative biasing source 26.

As regards the method of operation of the 25 above described circuit it will be appreciated that if the grid 22 of the modulating tube [6 is driven to a suitably positive potential relative to its cathode l7, then during a portion of each cycle of carrier wave frequency the voltage drop across the said tube It during the period in which its anode is positive relative to its cathode will be negligible because its impedance is low and the major part of the instantaneous potential generated in the oscillatory circuit i3, M will appear 35 across the load-circuit l8, I9 with little loss. Also the amplitude of the potential applied across the load circuit l8, l9 will depend upon the relation in time between the instantaneous values of potential in the oscillatory circuit l3, It and the moment at which the grid 22 becomes sufficiently positive for it to pass anode current, this amplitude varying from a maximum when the said moment corresponds with a high positive potential in the oscillatory circuit l3, M to zero when the said moment corresponds with zero or negative potential in the said oscillatory circuit 13, It will be seen therefore that the carrier power in the load circuit l8, l9 will be modulated as a result of varying the phase of the input potentials to the grid 22 of the modulating tube. The whole arrangement is made such that in the unmodulated or carrier condition the difference in phase between the anode and grid potentials on the modulating tube is suitably proportioned between the possible maximum and minimum values attained during modulation.

Also it will be appreciated that since the potentials in the oscillatory circuit I 3, i4 vary substantially according to a sine law it will be necessary, if it is desired, to cause the modulation of the load circuit current to vary in a rectilinear manner with the change of phase relationship between the anode and grid potentials, to adjust the initial phase displacement producing the unmodulated or carrier condition to give a somewhat large difference in phase between the anode and grid potentials of the modulating tube. If, for example, the initial phasing is such that the grid potential at 22 reached its maximum positive value at an instant when the oscillatory circuit (I3, I 4) positive potential has reached 15 of the eventual semi-cycle, then a variation of 15 leading or lagging on this mean position will cause the moment of maximum grid positive potential to occur at moments when the oscillatory circuit positive potential is either approximately twice the mean value or is zero, so that the variation of the load circuit current will be rectilinear in respect to the angular variation in phase.

Since only a rather small fraction of the maximum potential generated in the oscillatory circuit l3, I4 is actually employed to supply the load circuit l8, l9 it may be advantageous in certain cases to provide a step up transformer between the said oscillatory circuit i 3, I 4 and the series combination of load circuit l8, l9 and modulating tube H5, or alternatively to tap down" the anode 8 of the amplifier tube 6 on the inductance or capacity in its oscillatory circuit I 3, l4 so that the potential across the said oscillatory circuit is increased. In the embodiment illustrated the anode 8 is tapped down on the inductance i3 in the oscillatory circuit I 3, l4.

The circuit of Figure 2 is in general the same as the circuit of Figure 1. In Figure 2 the source of wave energy 2 is illustrated as being a tube generator comprising a tube V1 having its elec* trodes coupled in oscillation generating circuits. The wave energy produced in 2 is supplied from the oscillation circuit thereof to a tuned circuit Y and from said tuned circuit by way of coupling condenser I to the amplifier tube 6, as in Figure 1. The amplifier tube 6 is neutralized by a condenser NC connected as shown. This amplifier in other respects is quite similar to the one shown in Figure 1 as is its connections with the modulating stage Va, that is, tube I6, and needs no further description here. Wave energy from the oscillator 2 is also supplied by coupling X to the phase shifting circuit shown as a rectangle 2| in Figure 1 and shown more. in detail in.Figure 2. The phase shifting or adjusting means and/or modulating means comprises two tubes V4 and V5 having their control grids connected as shown by coupling condensers 34 and 38 respectively, to points on the phase shifting network R1 R2, L1 L2. The tubes V4 and V5 have their anodes connected in phase opposition to the secondary 48 of a transformer having its primary 46 coupled to the output electrodes of a modulation frequency amplifier 44, the input electrodes of which are connected by transformer 24 to a source of modulating potentials. The screen grid electrodes 38 and 40 of tubes V4 and V5 are connected to points on resistances 33 and 35 respectively connected with the secondary 48 of the transformer.

The carrier frequency oscillations are supplied to the control grids H and 42 of tubes V4 and Vs through an inductance L1 which serves to provide a suitable fixed phase difference between the potentials developed at the terminals of the network R1 L2 R: 01 and the potentials developed at the grid 4 of the amplifier stage V2 including tube 8. The control grid to cathode impedance of tube V4 is connected across the inductance 1a and the control grid to cathode impedance of V5 is connected across the condenser C1, resulting in a relative phase displacement between the potentials applied to the grids of the tubes V4 and V5.

On applying modulating potentials in phase opposition to the anodes of tubes V4 and V5 the oscillations produced in the common output circuit C2 L3 connected in phase to the anodes of tubes V4 and V5 will be varied in phase while maintaining substantially constant amplitude. These phase varied or modulated oscillations which vary at signal frequency are applied through coupling means Z to the control grid 22 of tube IS in stage V3 to produce the phase relation and consequent effect described hereinbei'ore.

In the arrangement illustrated, the load circuit .l8, l9 will be traversed by current only once in each full cycle of carrier wave frequency, but it is obviously possible so to arrange a pair of modulating tubes and a load circuit that the latter is traversed by current in each half cycle of carrier wave frequency. Such a full wave" arrangement is generally preferable.

Although in the illustrated embodiment modu lation is effected by varying the phase of the energy applied to the modulating tube, it is possible in carrying out the invention to maintain the positive potential on the grid of the modulating tube constant in amplitude and with unchanging phase relation to the potential across the series combination of load circuit and modulating tube (and preferably in phase equality therewith) and to vary in dependence upon modulating potentials by any convenient means, known per se, the time duration of the application of carrier potential to the modulating tube grid.

I claim:

1. In a signalling system, an electron discharge amplifier tube having input and output electrodes, means for impressing high frequency wave energy to be modulated on said input electrodes, a load circuit, an electron discharge device having output and input electrodes, means connecting said output electrodes of said device in series with said load circuit, means coupling said series circuit to the output electrodes of said first named tube whereby high frequency wave energy of substantially fixed phase is applied by said amplifier tube to said series circuit, an auxiliary electron discharge tube system having input electrodes and having output electrodes coupled to the input electrodes of said electron discharge device, a phase shifting circuit coupling said first means to the input electrodes of said last named tube system to impress thereon high frequency wave energy of a phase displaced relative to the phase of the high frequency wave energy supplied to said series circuit and means for modulating the impedance of said auxiliary electron discharge tube system at signal frequency to vary the phase of the high frequency wave energy supplied by said auxiliary tube system from said means to said input electrodes of said electron discharge device at signal frequency to thereby vary the impedance of said device in said series circuit and consequently vary the amplitude of the high frequency wave energy supplied to said load.

2. In a signalling system, an electron discharge amplifier tube having input and output electrodes, means for impressing high frequency wave energy to be modulated on said input electrodes, a load circuit, an electron discharge device having output and input electrodes, means connecting the output electrodes of said device in series with said load circuit, means coupling said series circuit to the output electrodes of said first named tube whereby high frequency wave energy of substantially fixed phase is applied by said amplifier tube to said series circuit, a pair of electron discharge tubes having input electrodes and having output electrodes coupled together and to the input electrodes of said second device, phase shifting circuits coupling said first means to the input electrodes of said last named pair of tubes for impressing voltages characteristic of said wave energy in phase displaced relation on said input electrodes, the mean phase of said impressed voltages being also displaced in phase relative to the wave energy applied to said series circuit, and means for difierentially modulating the impedance of said pair of tubes at signal frequency to vary the phase of the high frequency wave energy supplied by said pair of tubes to said input electrodes of said second named device at signal frequency to thereby vary the impedance of said device in said series circuit and consequently vary the amplitude of the high frequency wave energy supplied to said load.

3. In a modulation system, an electron discharge device having input and output electrodes coupled in high frequency alternating current input and output circuits, means for neutralizing said circuits, means for impressing high frequency wave energy to be modulated on said input circuit, a load circuit, an electron discharge device having input and output electrodes, means connecting the output electrodes of said second device in series with said loadcircuit, voltage step up means coupling said series circuit to the high frequency alternating current circuit connected with the output electrodes of said first named device whereby voltages characteristic of said high frequency wave energy are impressed by acteristic of said high frequency wave energy and of a phase which varies in accordance with said modulating potentials, between limits fixed by said phase shifting reactances, relative to the aforesaid voltages impressed on said series circuit.

4. In a modulation system, an electron discharge device having input and output electrodes coupled in high'frequency alternating current input and output circuits, means for neutralizing said circuits, means for impressing high frequencywave energy to be modulated on said input circuit, a load circuit, an electron discharge de-' vice having input and output electrodes, means connecting the output electrodes of said second device in series with said load and coupling the same to the alternating current circuit connected with the output electrodes of said first named device, a pair of electron discharge tubes each having input andoutput electrodes, phase shifting reactances connecting the input electrodes of said pair of tubes to said second named means, a source of modulating potentials connecting the output electrodes of said pair of tubes in phase oppositio and a. high frequency alternating current circuit coupling the output electrodes of said pair of tubes to the input electrode of said second named device.

5. A system as recited in claim 4 wherein said pair of tubes also each have an auxiliary electrade and said auxiliary electrodes are connected in phase opposition also by said source of modulating potential.

THEODORE DI'I'CHAM. 

